Unable to make/receive calls/texts
I am using a HTC One M8 with the provider BT, and have been for the last two years. As of about a week ago, for no obvious reason, I have become unable to make or receive calls and texts from this phone.
I have tried everything I can think of to fix the problem (even a full factory reset). When the 'Network Mode' is selected as '2G, 3G' the phone shows a connection to BT, 3G internet connection and a full mobile signal (even though unable to make calls etc). When the 'Network Mode' is changed to '2G, 3G, 4G Auto' the internet connection disappears and so does the mobile signal.
The SIM card appears to work fine in an older phone.
If the phone had been dropped, gotten wet or experienced any other form of potential damage, then I would assume it was a hardware repair issue. But because nothing has happened to obviously explain the issue I am experiencing, I am unsure.